The present invention relates to a process control technique and, more particularly, to a feedback control method and feedback control apparatus which perform set point tracking (follow-up) control by giving a manipulated variable to an object to be controlled such that the controlled variable tracks the set point.
PID control has conventionally been known as a highly practical general-purpose control theory. SAC (Simple Adaptive Control) is also known as an advanced control theory like the modern control theory. In either control theory, a manipulated variable MV is output as a control calculation result to an object to be controlled such that a controlled variable PV tracks a set point SP. The controlled variable PV is measured, and control calculation is performed based on a deviation Er from the set point SP.
General PID control is a linear control theory, and is a control theory which assumes that a control system including an object to be controlled is a linear system. An actual object to be controlled does not have strict linearity, and slight nonlinearity is permitted for PID control. Heating using a halogen lamp in RTP (Rapid Thermal Process) in a semiconductor manufacturing apparatus is executed by a highly nonlinear system to which PID control cannot be simply applied. In this case, even PID control can cope with tracking the stability of the control system. However, PID control cannot meet a condition under which high-speed temperature rise like RTP and a response waveform almost free from overshoot are required.
Assume that the nonlinearity of the control system can be approximated by a characteristic K in FIG. 14. If the temperature is increased at a high speed following the step change of the set point SP (step response), the manipulated variable MV (heating output) reaches 100% at a large deviation Er between the set point SP and the controlled variable PV. The average process gain characteristic changes to a characteristic Kav1 with a steep slope in FIG. 14. As the temperature rises and the deviation Er decreases, the manipulated variable MV drops to about 20%. Along with this, the average process gain characteristic changes to another characteristic Kav2 with a moderate slope in FIG. 14.
The PID parameters of a PID controller are adjusted in accordance with the high-speed temperature rise specifications, and the PID controller controls the temperature of a highly nonlinear system like the system in FIG. 14. The temperature rise locus (step response waveform) exhibits a characteristic PV in FIG. 15 with respect to the step change of the set point SP. More specifically, overshoot which controls an object with a large process gain occurs in the controlled variable PV in the first half of response. The second half of response suffers control operation in which tracking of the controlled variable PV for controlling an object with an excessively small process gain to the set point SP is extremely slow. The temperature rise locus as shown in FIG. 15 can be attained, but an object such as a semiconductor manufacturing apparatus which requires a response waveform almost free from overshoot is not properly controlled. Adjustment of the PID parameters is not defined by the linear control theory, and it is very difficult to adjust them.
An advanced adaptive control theory such as SAC is so designed as to automatically correct the internal parameters of a control calculation unit and always obtain a proper control characteristic with respect to variations in the process gain characteristic of an object to be controlled. For appropriate automatic correction (adaptive operation) of the internal parameters, control calculation must be executed a satisfactory number of times in the transient state. In high-speed temperature rise, the time necessary for temperature rise is about 1.0 to 1.5 sec, as shown in FIG 16A. For a control cycle of 50 msec, the control calculation count in the step response is about 20 to 30.
Under this condition, the control calculation count set to tracking a process gain change caused by the highly nonlinear characteristic is two or three at most, as shown in FIG. 16B. This control calculation count is insufficient to execute adaptive operation. In practical use, a method based on the advanced adaptive control theory can finally obtain the control stability at most, and cannot smoothly increase at a high speed the temperature of a highly nonlinear object to be controlled. This theory is substantially a technique for merely ensuring the stability regardless of high-speed temperature rise. As for many parameters which should be set in advance for proper adaptive operation, there is no practical use standard regarding settings.
As described above, when a highly nonlinear system is to be controlled, the conventional PID control theory cannot realize proper set point tracking control, and it is also difficult to adjust PID parameters.
In the advanced adaptive control theory such as SAC, the control calculation count is insufficient to execute adaptive operation when the controlled variable of a highly nonlinear object is made to tracking the set point at a high speed. Appropriate set point tracking control cannot be realized, and it is also difficult to adjust parameters.